As electronic devices, mainly various information terminals, have spread in quick tempo at offices and homes, there is a fear that static electricity and electromagnetic waves generated from those electronic devices may exert an influence on other electronic devices and the human body. In order to prevent troubles caused by such static electricity and electromagnetic waves, there now exists a demand for development of an electromagnetic wave shielding gasket having excellent electromagnetic wave shielding properties and flexibility. Further, under enforcement of the Product Liability Law (PL Law) for example, an electromagnetic wave shielding gasket satisfying the UL Standard, etc. and having a high degree of flame retardancy is keenly desired.
As an example of an electromagnetic wave shielding gasket there is known a gasket using a soft foaming material as a core material and fabric having electric conductivity wound and bonded onto the foaming material. Generally, for producing a soft electromagnetic wave shielding gasket, it is necessary that not only the core material but also the electrically conductive fabric wound round the core material be soft to a satisfactory extent. Moreover, for attaining a high degree of flame retardancy required of the electromagnetic wave shielding gasket, satisfactory flame retardancy is required for all of constituents used, including a core material, an electrically conductive fabric, and an adhesive for bonding the two. To meet this requirement, such compounds as halogen compounds, antimony compounds and phosphorus compounds have heretofore been used as flame retardants. For imparting a high degree of flame retardancy to the gasket in question there have been made various proposals of using those flame retardants in combination to obtain a synergistic effect. On the other hand, as recent products giving consideration to the environment, Electrically conductive fabrics using neither a halogen compound nor an antimony compound and yet having flame retardancy are being developed.
In JP 2003-243873A there is provided a gasket giving consideration to the environment and prepared by laminating a hot melt resin to the surface of an electrically conductive fabric, the hot melt resin containing a flame retardant other than halogen compounds and antimony compounds. The flame retardancy of this gasket satisfies V-1 of UL Standard 94, but does not satisfy V-0 that defies a higher degree of flame retardancy.
In JP 2003-258480A there is disclosed a flame-retardant, electromagnetic wave shielding material of a three-layer laminate structure comprising a filmy support, an electrically conductive thin film formed on one side of the filmy support and an electrically conductive, flame-retardant, adhesive layer. As a flame retardant there is used a phosphorus-based flame retardant which is a non-halogen type, and an electrically conductive powder is used for the purpose of imparting electrical conductivity to the adhesive layer. Such a flame-retardant electromagnetic wave shielding material is deficient in electrical conductivity of the filmy support although consideration is given to the environment. Further, even if material in question has flexibility against bending in only one direction, it is deficient in flexibility against three-dimensional deformations and is therefore unsuitable as an electromagnetic wave shielding gasket.
Further, a laminate produced by adding a flame retardant other than a halogen compound or an antimony compound to a general-purpose synthetic resin, laminating the synthetic resin containing the flame retardant to the surface of an electrically conductive fabric and further laminating thereto a hot-melt resin sheet is known as a material for a gasket. However, in the case of this material, since the number of laminate layers is large, the flexibility of the fabric is impaired and the number of processing steps increases.